The interaction force between the borehole wall and wireline tools or other downhole tools can become significant as a result of differential sticking phenomena. During open-hole wireline operations, the wellbore is typically pressurized above the formation pore pressure in order to prevent formation fluids from entering the wellbore. At such pressures drilling fluids may flow into permeable formations. Solid particles in the drilling fluids are often too large to enter the fine pore structure of the formation and remain on the borehole wall. These filtered particles are commonly referred to as mud cake or filter cake in the art.
When a wireline tool (or a drilling tool) contacts the mud cake, the fluid pressure on the borehole side of the tool often exceeds the fluid pressure on the formation side of the tool. This differential pressure may cause the tool to stick (or adhere) to the borehole wall. Such differential sticking can be problematic. For example, large axial forces are sometimes required to dislodge the tool from the borehole wall. In extreme cases, the magnitude of the force may exceed the maximum force that the wireline cable can carry. In such cases expensive and time consuming fishing operations (or other remedial operations) may be required to remove the tool from the wellbore.
There remains a need in the art for downhole tools that allow for easier retrieval in operations in which differential sticking is an issue.